Reductions of acetylenes

Hydrogenation. Acetylene can be hydrogenated to ethylene and ethane. The reduction of acetylene occurs in an ammoniacal solution of chromous chloride (20) or in a solution of chromous salts in H2SO4 (20). The selective catalytic hydrogenation of acetylene to ethylene, which proceeds... 

Stereospecifle reduction of acetylenic alcohols to E- aliyilc alcohols by means of sodium bls(2 melhoxyethoxy)aluminum hydride (SMEAH)... 

Epoxides are normally hydrogenated in preference to saturated ketones but double bonds are usually reduced under these conditions. It is possible in some cases to selectively cleave an epoxide without saturating double bonds by the use of the deactivated catalysts recommended for the partial reduction of acetylenes (see section IV) or by the addition of silver nitrate to the palladium-catalyzed reaction mixture. " ... 

Amines (7a,12a), especially pyridine (75), have also been used as solvents in the hydrogenation of acetylenes. Hydrogenation of 3 over 5% Pd-on-BaS04 in pyridine gave df-cis-jasmanate (4) quantitatively (40). The authors comment that this combination for reduction of acetylenes was superior to the Lindlar catalyst in all cases examined. (See also Refs. 12 and 24 for similar conclusions.)... 

Selective reduction of acetylenes containing carbonyl functions seems to present no difficulties if the groups are not conjugated. 

For reduction of acetylenic ketones, two oxidoreductases were used [25]. Lactobacillus brevis alcohol dehydrogenase (LBADH) gave the (R)-alcohols and Candida parapsilosis carbonyl reductase (CPCR) afforded the (S)-isomer, both in good yield and excellent enantioselectivity. By changing the steric demand of the substituents, the enantiomeric excess values can be adjusted and even the configurations of the products can be altered (Figure 8.34). 

There are reports (reviewed in Ref. (55)) of acetylene reduction by FeMoco using very powerful reductants such as sodium borohydride or sodium amalgam. Unfortunately, in most of these experiments the integrity of FeMoco after reaction was not established, and thus the reaction could have been carried out by fragments of FeMoco. Reduction of acetylene bound to a metal site by powerful reductants is well established. 

Reduction of acetylenes can be done with sodium in ammonia,220 lithium in low molecular weight amines,221 or sodium in HMPA containing /-butanol as a proton source,222 all of which lead to the A-alkene. The reaction is assumed to involve successive electron transfer and protonation steps. 

The stereochemistry of electrochemical reduction of acetylenes is highly dependent upon the experimental conditions under which the electrolysis is carried out. Campbell and Young found many years ago that reduction of acetylenes in alcoholic sulfuric acid at a spongy nickel cathode produces cis-olefins in good yields 126>. It is very likely that this reduction involves a mechanism akin to catalytic hydrogenation, since the reduction does not take place at all at cathode substances, such as mercury, which are known to be poor hydrogenation catalysts. The reduction also probably involves the adsorbed acetylene as an intermediate, since olefins are not reduced at all under these conditions and since hydrogen evolution does not occur at the cathode until reduction of the acetylene is complete. Acetylenes may also be reduced to cis olefins in acidic media at a silver-palladium alloy cathode, 27>. 

Asymmetric Reduction of Acetylenic Ketones with Amino Alcohol-LAH Reagents... 

Brinkmeyer and Kapoor (101) reported that the chiral hydride complex formed from LAH and (+ )-90 (Darvon alcohol) gave high enantiomeric ratios of chiral propargylic carbinols in the reduction of acetylenic ketones (Table 10, entries... 

Chromium(II) sulfate is a versatile reagent for the mild reduction of a variety of bonds. Thus aqueous dimethylformamide solutions of this reagent at room temperature couple benzylic halides, reduce aliphatic monohalides to alkanes, convert vicinal dihalides to olefins, convert geminal halides to carben-oids, reduce acetylenes to /raw5-olefins, and reduce a,j3-unsatu-rated esters, acids, and nitriles to the corresponding saturated derivatives. These conditions also reduce aldehydes to alcohols. The reduction of diethyl fumarate described in this preparation illustrates the mildness of the reaction conditions for the reduction of acetylenes and o ,j8-unsaturated esters, acids, and nitriles. 

Palladium catalyst for partial reduction OF ACETYLENES, 46, 89 Palladium on charcoal, catalyst for reductive methylation of ethyl p-. nitrophenylacetate, 47, 69 in reduction of /-butyl azidoacetate to glycine /-butyl ester, 46, 47 Palladium oxide as catalyst for reduction of sodium 2-nltrobenzene-sulfinate, 47, 5... 

Stereoselective reduction of acetylenic alcohols to it-allylic alcohols using sodium bis(2-methoxyethoxy)aluminum hydride (SMEAH, also known as Red-Al) or Li-AIH4. 

The reduction of acetylenic tellurides to the vinylic ones is achieved by treatment with NaBH4 in EtOH. The formal unusual reduction of the carbon triple bond by NaBH4 can be rationalized involving the attack of a hydride ion to the tellurium atom producing a tellurol and an acetylenic anion followed by the addition of the tellurolate anion to the acetylene, DIBAL-H has later been employed as a reducing agent. ... 

Another highly active non-pyrophoric nickel catalyst is prepared by reduction of nickel acetate in tetrahydrofuran by sodium hydride at 45° in the presence of tert-amyl alcohol (which acts as an activator). Such catalysts, referred to as Nic catalysts, compare with P nickel boride and are suitable for hydrogenations at room temperature and atmospheric pressure, and for partial reduction of acetylenes to civ-alkenes [49]. 

Reduction of 3-trimethyls1lyl-2-propyn-l-ol exemplifies the problem of stereoselectivity in hydride reduction of acetylenic alcohols to E-allyl alcohols.4 Early reports5 that lithium aluminum hydride stereoselectively reduced acetylenic alcohols gave way to closer scrutiny which revealed a striking solvent dependence of the stereochemistry. Specifically, the... 

Among the important reagents for which preparative procedures are given are 2,2 -bipyridine (by nickel directed and catalyzed dehydrogenation of pyridine p. 5), formamidine acetate (p. 39), phenyltrichloromethylmercury (p. 98), and trimethyl- and triethyloxonium fluoroborate (pp. 120, 113). The preparation of palladium catalyst ( Lindlar ) for the selective reduction of acetylenes is described (p. 89), as is the use of di-phenyliodonium-2-carboxylate, as a precursor of benzyne in the synthesis of 1,2,3,4-tetraphenylnaphthalene (p. 107). 

Nitrogenase catalyzes the reduction of a number of substrates in addition to N2 and H+. Acetylene is reduced to ethylene, and nitrous oxide to dinitrogen, while azide undergoes reduction to N2 and NH3. In the last two cases the product N2 is not reduced further, implying that it is not in the correct binding position for reduction. The presence of multiple sites on the MoFe protein suggested by this observation is also supported by the non-competitive nature of the inhibition shown by some of these compounds. The reduction of acetylene has been used as a marker reaction for nitrogenase activity. 

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